The present invention relates to substrate holder assemblies that retain a substrate during chemical-mechanical polishing (sometimes referred to as "CMP"). More particularly, the present invention relates to substrate holder assemblies, which include shims or shims secured by modified backing plates and retain a substrate during chemical-mechanical polishing (CMP) to produce a more uniformly polished substrate surface.
Chemical mechanical polishing (CMP) typically involves mounting a wafer faced down on a holder and rotating the wafer face against a polishing pad mounted on a platen, which in turn is rotating or in an orbital state or in linear motion. A slurry containing a chemical that chemically interacts with the facing wafer layer and an abrasive that physically removes that layer is flowed between the wafer and the polishing pad or on the pad near the wafer. In semiconductor wafer fabrication, this technique is commonly applied to polish various wafer layers such as dielectric layers, metallization, etc.
FIG. 1 shows some major components of a representative substrate holder assembly 10 that are currently integrated into a CMP apparatus. A shaft 12 lowers substrate holder assembly 10 holding a substrate 20 on a polishing pad 22 to begin CMP. Substrate holder assembly 10 includes a backing plate 14 having a convex or outwardly protruding dome shaped contact surface 15. A carrier film 18 adheres to contact surface 15 and thereby substantially conforms to protruding dome shape of the contact surface. A circumferential restraint member 16 engages the edges of substrate 20 and restrains the movement of substrate 20 outside substrate holder assembly 10. In this configuration, the protruding dome shape of contact surface 15 applies more pressure at a center region than peripheral regions of substrate 20 during CMP.
It is important to note that contact surface 15 is normally substantially planar when the substrate holder assembly is commercially obtained from manufacturers of the substrate holder assemblies, such as Integrated Processing Equipment Corporation (IPEC) of Phoenix, Ariz. At an additional cost, however, manufacturers of substrate holder assemblies may machine the contact surface to have a protruding dome shape as shown in FIG. 1 and according to specifications provided by an end user, e.g., an integrated circuit fabrication facility.
An end user may desire a contact surface having a protruding dome shaped region to effectively combat "center slow" polishing experienced by a substrate. It is well known in the art that "center slow" polishing refers to the condition when a film removal rate at a center region of a substrate surface is slower relative to the edge or peripheral regions of the substrate surface. Those skilled in the art recognize that center slow polishing conditions may set in when a polishing pad surface degrades due to repeated mechanical action of the substrate on the polishing pad during CMP. A substrate surface may suffer from center slow polishing well before the end of a production lot draws near. "Production lot" refers to a collection of substrates that are fabricated as a group under substantially similar conditions and may ultimately be sold.
Center slow polishing is undesirable because it leads to a non-uniformly polished wafer surface, i.e. the center region of the substrate surface is not polished to the same extent as the peripheral region of the substrate. In order to prevent forming non-uniformly polished substrate surfaces, the polishing pad with the degraded surface is typically replaced with a new polishing pad and the life of the polishing pad ends prematurely. In a typical integrated circuit (e.g., semiconductor wafer) fabrication facility, where several CMP apparatus are employed, the replacement cost of polishing pads can be significant.
Unfortunately, the current substrate holder assembly designs suffer from several drawbacks. By way of example, machining the contact surfaces mentioned above within the tolerances required by the end user can be an arduous, time-consuming and expensive task. Generally highly skilled workers, who may undergo extensive training, are required to perform precise machining of the contact surface. The end users, nevertheless, frequently discover that the dimensions of the protruding dome shape of the contact surface are not within the requisite tolerance levels to effectively combat center slow polishing.
Even in those instances where the dimensions of the backing plate comply with the end user's specification, the dimensions that were once deemed appropriate by the end user may no longer be suitable due to the changing parameters of the CMP system. For example, as substrate CMP proceeds through a production lot, the compressibility of the carrier film may change and/or the polishing pad characteristics are altered. In other words, the degree of center slow polishing changes as substrate CMP proceeds through a production lot. Consequently, the degree of protrusion of the dome shaped contact surface required to combat center slow polishing changes accordingly.
As another example, the protruding dome shaped of the contact surface is susceptible to undergoing deformation, e.g., formation of nicks, indentations and the like, during polishing of a production lot. The end users, therefore, may be forced to maintain several substrate holder assemblies including backing plates having varying degrees of dome shaped protrusion ready for operation in the event a backing plate is deformed or a backing plate having a different degree of dome shaped protrusion is necessary to combat the changing degree of center slow polishing.
It is well known in the art that maintaining several such substrate holder assemblies is expensive for the end user. Furthermore, the other drawbacks of the current substrate holder assembly design mentioned above translate into a lower throughput and yield of the CMP process.
What is therefore needed is an improved substrate holder assembly that facilitates in producing a more uniformly polished substrate surface, without appreciably lowering the yield or throughput of the CMP process.